a) Field of the Invention
This invention relates to dispersants for organic pigments (hereinafter simply referred to as "pigments"), pigment dispersions and writing or recording pigment inks, and more specifically to pigment dispersants and also to pigment dispersions making use of the pigment dispersants, having excellent fluidity, dispersion stability, storage stability and the like and suited especially for the production of writing or recording pigment inks.
b) Description of the Related Art
Marking inks for use in writing instruments, which make use of bundled fiber tips or felt tips, have conventionally been made of resins, pigments and solvents, and as the pigments, dyes have been employed. As the solvents, ester or aromatic solvents have been used from the standpoint of solubility for the resins and also solubility for the dyes.
It is however desired to avoid use of aromatic solvents, because they give deleterious effects on the health of workers (organic solvent intoxication) and they themselves are air pollutants. Although there is a move toward water-based writing inks, solvent-based maker pens are still employed widely. This can be attributed to the excellent writing performance of solvent-based inks on plastic films such as polyethylene films and also to the superb drying characteristics of the solvent-based inks shortly after written on plastic films.
Ink-jet printing, on the other hand, is a digital printing controlled by a computer. Printing information signals are supplied to a printer directly from the computer, so that no plate-making is needed. Ink-jet printing is therefore suited particularly for the printing of various images in small numbers of copies, and recent advancements in ink-jet printers have made it possible to perform printing of a variety of highly-detailed large images.
Ink-jet printing inks are required to have low viscosity and excellent stability. There are organic-solvent-based inks making use of dyes, but as pigment-type inks, water-based inks are used because ink-jet printing inks making use of pigments involve technical difficulties. However, water-based inks contain, as vehicles, resins at extremely low concentrations so that good color developments are not available. Water-based inks are also accompanied by a problem of adhesion to plastic films. Under these circumstances, there is a strong desire for the development of alcohol-based ink-jet printing inks making use of pigments and assuring good color developments.
Incidentally, as is observed from paints and the like, the conventional technology expects much on the dispersing power of a resin for a pigment upon dispersing the pigment on a resin solution. When the dispersing power of the resin is insufficient for the pigment, a pigment dispersant (pigment treatment) (hereinafter simply referred to as "a pigment dispersant) has been employed. in general paints, sufficient dispersion of pigments is feasible with conventional dispersants.
Compared with paints, however, writing or recording pigment inks are required to have extremely low viscosity and in addition, to have an extremely high degree of dispersion of pigments. Despite these requirements, use of conventional resins and dispersants is unable to achieve sufficient dispersion of pigments or leads to changes in the viscosity of pigment dispersions along the passage of time due to desorption of the dispersant from the pigments and insufficient compatibility between the pigments and the resins, thereby making it extremely difficult to obtain pigment dispersions which meet the property and performance requirements.
If an alcoholic solvent such as ethanol can be used as a solvent in an organic-solvent-based pigment ink, deleterious effects on the health of users of writing instruments and people studying or working in the same environment as the users can be reduced, the problem of air pollution can be lessened owing to the avoidance of an organic solvent, and further, the problems of drying characteristics and wetting to plastic films, said problems being inherent to water-based inks, can also be eliminated. Moverover, alcoholic solvents are resources reproducible in the natural world so that use of such solvents is preferred. Nonetheless, with resins soluble in conventionally-known alcoholic solvents, it is still extremely difficult to obtain pigment inks satisfactory in the requirements for low viscosity, high dispersion and high dispersion stability even if dispersants are used.